Below are two opinions on Brinell Hardness Number (BHN). Both are from Lead Bullet Casting experts, both express a different opinion on the subject of BHN.
-- First, Missouri Bullet Company - from the company President, though his name is not listed
Most cast bullet shooters don't know a lot about the properties of the lead alloy they're shooting because they haven't been educated about it. If you want to learn a little bit about some important cast bullet facts, then please read on.
A common conception is that when it comes to lead bullets, harder lead equals less leading. This is a false perception! To explain this surprising statement, it is necessary to discuss the physics of getting the bullet out of the barrel and how lead residue comes to be deposted in the bore. When the powder charge ignites, pressure is generated. This pressure is measured in “copper units of pressure” (CUP) and expressed in thousand of pounds per square inch. The heavier the powder charge, the greater the CUP. Naturally, the purpose of generating pressure in the cartridge case is to force the bullet out of the case mouth and on down the barrel.
Lead is a soft metal. Its hardness is expressed on a standard scale, called the Brinell Hardness Number (BHN.) The BHN of the bullet interacts with the pressure generated by the burning powder. The mechanism of this involves the effect of the generation of thousands of pounds per square inch of pressure which causes the base of the bullet to expand, or “obturate”. Properly obturated, the base will have expanded beyond its original diameter which has the effect of “sealing the bore” against the explosive pressure of the gases burning behind it. Properly sealed, and working in conjunction with the lubricant in the lube groove, the bullet will thus not allow gases to escape forward from around the base of the bullets, which prevents it from shaving lead from the bullet body and forcing it into the bore grooves (otherwise known as “leading”.)
This failure to obturate (“seal the bore against onrushing gases”) causes leading which is a chore to clean and is a major obstacle to accuracy.
An optimally hard lead bullet is simply one which obturates at a given pressure sufficiently to seal the bore against the gases which would otherwise “cut through” the soft lead (called “gas-cutting”, forcing molten lead into your rifling. A bullet which is too hard won't obturate and seal the bore, because the gas pressure is insufficient to expand the base of the bullet. A bullet which is too soft at a given pressure will experience excessive base expansion and vaporization of the lead, causing leading.
There is a formula for optimal bullet hardness which is simple and it is worth knowing:
Optimum BHN = CUP / (1422 x .90)
The CUP of your reloads is published in the reloading manuals. Take a typical .45 ACP load, using a 200-grain LSWC bullet – 5.0 grains of Bullseye. This load develops 900 FPS and is in common use among IPSC and IDPA gunners. The reloading manual shows that the pressure generated by this load is 20,000 CUP. So, the formula for optimal bullet hardness is
20,000 / 1279.8 = 15.62
There it is! For this application – shooting a 200-grain LSWC at 900 FPS requires that you use a bullet with a BHN of 16 to 18 (round upwards a couple of BHN points for flexibility.)
You may be asking why shooters don't know much about this whole bullet hardness optimization business. The reason is basically that the large manufacturers, for ease of production, use a standard alloy for all of their cast bullet construction, an alloy which has a Brinell Hardness Number of approximately 24. Why do they do this? It's simple – one standard alloy simplifies logistics for the big manufacturers and, equally importantly, a bullet this hard ships well by standing up to getting dinged around during transportation. The fact that their bullets are too hard and cause leading and aren't very accurate because of improper obturation is something they'd really rather you weren't aware of. This explains why neither their packaging nor product information will ever refer to the BHN of their products.
Along those lines, how many boxes of cast bullets – from any source – state the BHN on them?
At the Missouri Bullet Company, we optimize bullets for your intended application. We don't take a “one size fits all” approach to manufacturing your bullets. Every box of our bullets displays the BHN, which we constantly sample and monitor. We take the time to create lead bullet alloy that is specialized for the bullet hardness that works best for you. It is a fact that we spend significantly more time alloying our lead than we do in making the bullets that come from it and we do this to provide you the right bullet for your application.--
Second opinion is from Bob Palermo, President of Penn Bullets
BHN, Optimal Hardness, Leading Issues And Other BS
The use of BHN numbers has become a standard among consumers to compare and contrast cast bullets from various manufacturers to quantify the differences among the various cast bullets available to the consumer. Supposedly by understanding how hard a bullet is, the consumer can then select the right hardness for his or her particular shooting application.
I don’t as rule like using BHN numbers and when customers call asking for the BHN numbers of our bullets I want to take way more time to explain what they need to know rather than what they think they need to know.
The problem is that BHN is only one measured aspect of a cast bullets alloy. Not understanding what that number really means or what it signifies is really meaningless. Some have offered pressure equations that allow one to determine the amount of hardness needed for a particular load and a given velocity so the consumer feels he has chosen the optimal bullet for his needs. These equations show how much pressure is needed to make the bullet base expand (obturate) in order for the bullet to fill the lands and grooves of a barrel. Such a need was required in the old days especially with old black powder rifles and pistols and barrel dimensions that were far from being exact and consistent from gun maker to gun maker. Even barrels made by the same maker had wildly different tolerances.
It’s kind of funny that despite the vast improvements in CNC machining technology that we still find wide variance even today in some guns. For example, I like to pick on Ruger as one of those companies that has produced great guns but also has given us some horrible dimensions in those guns. Older .45 Colt guns had cylinder throats that would routinely measure in the plus .455" - .457" range (I know as I have one). At the same time the current versions of that gun are running cylinder measurements of .451" (I have one of those too) and then over a period of time the cylinders went to .454" as reported by customers who bought later versions of the Vaquero. Recently in the last few months there were two S&W revolvers that had cylinder throats at .432" and that is rare as most of the S&W revolvers in .44 mag. were just fine at .430" on the cylinder throats. By the same token the Rugers and the S&W guns have been fairly good on barrel dimensions with most coming in at standard specs for a given caliber. Colt Pythons have had some of the tightest barrel dimensions that I have measured at .354". The point is that within a range the tolerances have been somewhat consistent thru the runs of these guns with most of the variations coming in the cylinder throats. Now once those dimensions are known for your particular gun then the issue of getting bullets of a proper fit is relatively easy. Once you get bullets of a proper fit then obturation no longer becomes necessary to get an effective seal. If the bullet is at or over the largest dimension then the bullet will swage down and seal and make a perfect fit. The only time this becomes a problem is if the throat dimensions are off by a large margin of more than .002"-.003" over the barrel dimensions. Much more than that can cause the bullet to strip away within the barrel leaving long strings of leading down the barrel.
Now what does this have to do with BHN numbers? Glad you asked.
BHN numbers only tell you how hard something is period. It tells you nothing about Alloy Strength. I can easily show you two bullets that will both measure for all intents and purposes the same BHN numbers but be wildly different in the alloy composition. One will be hard and brittle with high exposure of antimony in the mix and very prone to lead fouling while the other with an equally high number will be much stronger and far more ductile in its strength. The standard 2/6 alloy (2% tin, 6 % antimony) as developed by Taracorp wasn’t developed for its performance characteristics as a cast bullet alloy but rather its performance as an alloy that would pour and run well in automatic casting machines that produced cast lead bullets. The 2/6 alloy proved to be a good alloy for a range of shooting applications but also had problems of being too hard and brittle for some applications and not strong enough for other applications.
Let us take a look at two other alloys. Linotype which is a 4/12 mix (4% Tin, 12% antimony and the balance lead) that is also known as a eutectic alloy. This simply means that this alloy at these ratios will melt and solidify at the same temperature. The alloy is hard and strong and measures about 21-22 on the BHN scale. It was the standard alloy as used in old line-o-type machines as used by printers and newspapers all over the world. Great stuff but it’s almost non-existent in the USA as offset and computer printing took over those tasks.
The alloy in straight form was usually too hard for most cast bullet users except for rifle use where its performance up to 2100 fps could be had without the need for gas checks on the base of the bullets. Most casters would use type metal for hardening a supply of softer lead and in the beginning this was the method I used to produce my own alloys and still use some of these materials to this day as much as the supply of those items will permit.
The other alloy is Lyman’s No. 2 alloy which Lyman developed many years ago. It is essentially a 5/5 alloy with a 5% tin and 5% antimony mix, balance lead. This alloy which is expensive to produce in large quantities found a home with the hobbyist bullet caster as its performance was very good. This one to one ratio provided an alloy that while it did not measure as high as the 2/6 alloy on the BHN scale had the advantage of being almost as strong as the linotype alloy (4/12).
That’s why I tell people not to get hung up on BHN numbers because they can’t tell you the whole story and using them as a comparison between cast bullet brands is pretty much worthless without having all the information at your disposal.
The advantage that I have is that by having my own foundry and mixing my own metals for my own use in my products is that I have total control over the QUALITY and STRENGTH of the alloys that I use. The alloys that I offer because of their high strength are more adaptable and usable over a much wider range of loads and velocities than other casters that buy and use the standard 2/6 mix.